Standing wave nuclear fission reactor and methods

ABSTRACT

Disclosed embodiments include nuclear fission reactor cores, nuclear fission reactors, methods of operating a nuclear fission reactor, and methods of managing excess reactivity in a nuclear fission reactor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims the benefit of theearliest available effective filing date from the following listedapplication (the “Related Application”) (e.g., claims benefits under 35USC §119(e) for provisional patent applications, for any and all parent,grandparent, great-grandparent, etc. applications of the RelatedApplication).

RELATED APPLICATIONS

For purposes of the USPTO extra-statutory requirements, the presentapplication claims benefit of priority of U.S. Provisional PatentApplication No. 61/280,370 , entitled TRAVELING WAVE NUCLEAR FISSIONREACTOR FUEL SYSTEM AND METHOD, naming Charles E. Ahlfeld, Thomas M.Burke, Tyler S. Ellis, John Rogers Gilleland, Jonatan Hejzlar, PavelHejzlar, Roderick A. Hyde, David G. McAlees, Jon D. McWhirter, AshokOdedra, Robert C. Petroski, Nicholas W. Touran, Joshua C. Walter, KevanD. Weaver, Thomas Allan Weaver, Charles Whitmer, Lowell L. Wood, Jr.,and George B. Zimmerman as inventors, filed Nov. 2, 2009 , which wasfiled within the twelve months preceding the filing date of the presentapplication or is an application of which a currently co-pendingapplication is entitled to the benefit of the filing date.

The United States Patent Office (USPTO) has published a notice to theeffect that the USPTO's computer programs require that patent applicantsreference both a serial number and indicate whether an application is acontinuation or continuation-in-part. Stephen G. Kunin, Benefit ofPrior-Filed Application, USPTO Official Gazette Mar. 18, 2003 ,available athttp://www.uspto.gov/web/offices/com/sol/og/2003/week11/patbene.htm. Thepresent Applicant Entity (hereinafter “Applicant”) has provided above aspecific reference to the application(s) from which priority is beingclaimed as recited by statute. Applicant understands that the statute isunambiguous in its specific reference language and does not requireeither a serial number or any characterization, such as “continuation”or “continuation-in-part,” for claiming priority to U.S. patentapplications. Notwithstanding the foregoing, Applicant understands thatthe USPTO's computer programs have certain data entry requirements, andhence Applicant is designating the present application as acontinuation-in-part of its parent applications as set forth above, butexpressly points out that such designations are not to be construed inany way as any type of commentary and/or admission as to whether or notthe present application contains any new matter in addition to thematter of its parent application(s).

All subject matter of the Related Application and of any and all parent,grandparent, great-grandparent, etc. applications of the RelatedApplication is incorporated herein by reference to the extent suchsubject matter is not inconsistent herewith.

BACKGROUND

The present patent application relates to nuclear fission reactors andmethods.

SUMMARY

Disclosed embodiments include nuclear fission reactor cores, nuclearfission reactors, methods of operating a nuclear fission reactor, andmethods of managing excess reactivity in a nuclear fission reactor.

The foregoing is a summary and thus may contain simplifications,generalizations, inclusions, and/or omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is NOT intended to be in any way limiting. Inaddition to any illustrative aspects, embodiments, and featuresdescribed above, further aspects, embodiments, and features will becomeapparent by reference to the drawings and the following detaileddescription. Other aspects, features, and advantages of the devicesand/or processes and/or other subject matter described herein willbecome apparent in the teachings set forth herein.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1C are partial-cutaway perspective views of an illustrativenuclear fission reactor.

FIG. 2 is a top plan view in schematic form of an illustrative nuclearfission reactor core.

FIG. 3 is partial-cutaway perspective view in schematic form of anillustrative nuclear fuel assembly.

FIG. 4A is a partial-cutaway perspective view in schematic form ofillustrative fuel assembly flow receptacles.

FIG. 4B illustrates a graph of relative flux distribution overlaid witha side plan view in schematic form of an illustrative stepped coresupport grid plate.

FIGS. 5A and 5B are side plan views in schematic form of illustrativedecay heat removal systems.

FIGS. 6A and 6B are illustrative graphs of reactivity versus burnup.

FIG. 7 is an illustrative graph of plutonium isotope evolution versusutilization of U²³⁸.

FIG. 8A is a flowchart of an illustrative method of operating a nuclearfission reactor.

FIGS. 8B-8Y are flowcharts of illustrative details of the method of FIG.8A.

FIG. 9A is a flowchart of another illustrative method of operating anuclear fission reactor.

FIGS. 9B-9W are flowcharts of illustrative details of the method of FIG.9A.

FIG. 10A is a flowchart of an illustrative method of managing excessreactivity in a nuclear fission reactor.

FIGS. 10B-10H are flowcharts of illustrative details of the method ofFIG. 10A.

DETAILED DESCRIPTION

Introduction

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings, theuse of similar or the same symbols in different drawings typicallyindicates similar or identical items, unless context dictates otherwise.

The illustrative embodiments described in the detailed description,drawings, and claims are not meant to be limiting. Other embodiments maybe utilized, and other changes may be made, without departing from thespirit or scope of the subject matter presented here.

One skilled in the art will recognize that the herein describedcomponents (e.g., operations), devices, objects, and the discussionaccompanying them are used as examples for the sake of conceptualclarity and that various configuration modifications are contemplated.Consequently, as used herein, the specific exemplars set forth and theaccompanying discussion are intended to be representative of their moregeneral classes. In general, use of any specific exemplar is intended tobe representative of its class, and the non-inclusion of specificcomponents (e.g., operations), devices, and objects should not be takenlimiting.

The present application uses formal outline headings for clarity ofpresentation. However, it is to be understood that the outline headingsare for presentation purposes, and that different types of subjectmatter may be discussed throughout the application (e.g.,device(s)/structure(s) may be described under process(es)/operationsheading(s) and/or process(es)/operations may be discussed understructure(s)/process(es) headings; and/or descriptions of single topicsmay span two or more topic headings). Hence, the use of the formaloutline headings is not intended to be in any way limiting.

Overview

Referring now to FIGS. 1A-1C and FIG. 2 and given by way of non-limitingoverview, an illustrative nuclear fission reactor 10 will be describedby way of illustration and not of limitation. As will be discussed belowin detail, embodiments of the nuclear fission reactor 10 arebreed-and-burn fast reactors (also referred to as traveling wavereactors, or TWRs) in which a standing wave of breeding-and-fissioning(also referred to as a breed-burn wave) via movement (also referred toas shuffling) of nuclear fuel assemblies.

Still by way of overview, a nuclear fission reactor core 12 is disposedin a reactor vessel 14. A central core region 16 (FIG. 2) of the nuclearfission reactor core 12 includes fissile nuclear fuel assemblies 18(FIG. 2). The central core region 16 also includes fertile nuclear fuelassemblies 20 a (FIG. 2). The central core region 16 also includesmovable reactivity control assemblies 22 (FIG. 2).

A peripheral core region 24 (FIG. 2) of the nuclear fission reactor core12 includes fertile nuclear fuel assemblies 20 b (FIG. 2). It will beappreciated that the fertile nuclear fuel assemblies 20 a and 20 b maybe made of the same or similar construction (as indicated by use ofsimilar reference numbers). As will be explained further below, thefertile nuclear fuel assemblies 20 a reside in a neutron fluxenvironment in the central core region 16 that is different from theneutron flux environment in the peripheral core region 24 (in which thefertile nuclear fuel assemblies 20 b reside). As a result, over corelife the fertile nuclear fuel assemblies 20 a may undergo breeding andmay experience burnup at rates that are different from rates undergoneand experienced by the fertile nuclear fuel assemblies 20 b. Therefore,the similar (but not the same) reference numbers 20 a and 20 b are usedto help keep track of the fertile nuclear fuel assemblies 20 a and 20 bduring discussions herein of various phases of core life. The peripheralcore region 24 also includes neutron absorber assemblies 26.

An in-vessel handling system 28 is configured to shuffle ones of thefissile nuclear fuel assemblies 18 and ones of the fertile nuclear fuelassemblies 20 a and 20 b. The nuclear fission reactor 10 also includes areactor coolant system 30.

Continuing by way of non-limiting overview, according to some aspectsmethods are provided for operating a nuclear fission reactor. Given byway of non-limiting example, in some embodiments fissile nuclear fuelmaterial in a plurality of fissile nuclear fuel assemblies is fissionedin a central core region of a nuclear fission reactor core of a nuclearfission reactor. Fissile material is bred in ones of a plurality offertile nuclear fuel assemblies in the central core region of thenuclear fission reactor core, and selected ones of the plurality offissile nuclear fuel assemblies and selected ones and selected others ofthe plurality of fertile nuclear fuel assemblies are shuffled in amanner that establishes a standing wave of breeding fissile nuclear fuelmaterial and fissioning fissile nuclear fuel material.

Continuing by way of non-limiting overview, according to some aspectsmethods are provided for managing excess reactivity in a nuclear fissionreactor. Given by way of non-limiting example, in some embodimentscriticality with a positive quantity of reactivity is achieved in acentral core region of a reactor core of a nuclear fission reactor. Thequantity of reactivity is increased until a predetermined burnup levelis achieved in selected ones of fuel assemblies in the reactor core, andthe increase in reactivity is compensated for.

Details will be set forth below by way of non-limiting examples.

Illustrative Nuclear Fission Reactors

In the discussion set forth below, details regarding extra-corecomponents of the nuclear fission reactor 10 will be set forth first byway of non-limiting examples. Details regarding extra-core components ofthe nuclear fission reactor 10 will be set forth next by way ofnon-limiting examples. This ordering of discussion details willfacilitate an understanding of establishment of a standing wave ofbreeding and fissioning in the nuclear fission reactor core 10.

Extra-Core Components

Still referring to FIGS. 1A-1C and FIG. 2, embodiments of the nuclearfission reactor 10 may be sized for any application as desired. Forexample, various embodiments of the nuclear fission reactor 10 may beused in low power (around 300 MW_(e)—around 500 MW_(e)) applications,medium power (around 500 MW_(e)—around 1000 MW_(e)) applications, andlarge power (around 1000 MW_(e) and above) applications as desired.

Embodiments of the nuclear fission reactor 10 are based on elements ofliquid metal-cooled, fast reactor technology. For example, in variousembodiments the reactor coolant system 30 includes a pool of liquidsodium disposed in the reactor vessel 14. In such cases, the nuclearfission reactor core 12 is submerged in the pool of sodium coolant inthe reactor vessel 14. The reactor vessel 14 is surrounded by acontainment vessel 32 that helps prevent loss of sodium coolant in theunlikely case of a leak from the reactor vessel 14.

In various embodiments the reactor coolant system 30 also includesreactor coolant pumps 34. The reactor coolant pumps 34 may be anysuitable pump as desired, such as for example electromechanical pumps orelectromagnetic pumps.

In various embodiments the reactor coolant system 30 also includes heatexchangers 36. The heat exchangers 36 are disposed in the pool ofprimary liquid sodium. The heat exchangers 36 have non-radioactiveintermediate sodium coolant on the other side of the heat exchangers 36.To that end, the heat exchangers 36 may be considered intermediate heatexchangers. Steam generators (not shown for clarity in FIGS. 1A-1C and2) are in thermal communication with the heat exchangers 36. It will beappreciated that any number of reactor coolant pumps 34, heat exchangers36, and steam generators may be used as desired.

The reactor coolant pumps 34 circulate primary sodium coolant throughthe nuclear fission reactor core 12. The pumped primary sodium coolantexits the nuclear fission reactor core 12 at a top of the nuclearfission reactor core 12 and passes through one side of the heatexchangers 36. Heated intermediate sodium coolant is circulated viaintermediate sodium loops 42 to the steam generators (not shown) that,in turn, generate steam to drive turbines (not shown) and electricalgenerators (not shown).

During periods of reactor shut down, in some embodiments plantelectrical loads are powered by the electrical grid and decay heatremoval is provided by pony motors (not shown for clarity) on thereactor coolant pumps 34 that deliver reduced reactor coolant flowthrough heat transport systems.

Referring additionally to FIGS. 5A and 5B, in various embodiments thenuclear fission reactor 10 includes a decay heat removal system 38. Inthe event that electrical power is not available from the electric grid,decay heat is removed using the decay heat removal system 38. In variousembodiments, the decay heat removal system 38 may include either one orboth of two dedicated safety class decay heat removal systems 38 a (FIG.5A) and 38 b (FIG. 5B) which operate entirely by natural circulationwith no need for electrical power. In the safety class decay heatremoval system 38 a (FIG. 5A), heat from the nuclear fission reactorcore 12 first is transferred by naturally circulated sodium to thereactor vessel 14, then is radiated across an argon-filled gap 40between the reactor vessel 14 and the containment vessel 32, and finallyis removed by naturally circulating ambient air that flows along thewall of the containment vessel 32.

In the safety class decay heat removal system 38 b (FIG. 5B), the heatexchangers 36 and the intermediate sodium loops 42 (FIGS. 1A-1C)transfer heat by natural circulation of sodium to steam generators 44where the heat is dissipated through shell walls of the steam generator44 using ambient temperature air drawn in through protected air intakes46.

Referring back to FIGS. 1A-1C and 2, the in-vessel handling system 28 isconfigured to shuffle ones of the fissile nuclear fuel assemblies 18 andones of the fertile nuclear fuel assemblies 20 a and 20 b. In somestages of core life (as will be discussed below), it may be desired toshuffle ones of the fissile nuclear fuel assemblies 18 and ones of thefertile nuclear fuel assemblies 20 a and 20 b between the central coreregion 16 and the peripheral core region 24. Thus, the in-vesselhandling system 28 may also be configured to shuffle ones of the fissilenuclear fuel assemblies 18 and ones of the fertile nuclear fuelassemblies 20 a and 20 b between the central core region 16 and theperipheral core region 24.

It will be appreciated that the in-vessel handling system 28 permitsmovement of the selected fissile nuclear fuel assemblies 18 and fertilenuclear fuel assemblies 20 a and 20 b without removing the moved fissilenuclear fuel assemblies 18 and fertile nuclear fuel assemblies 20 a and20 b from the nuclear fission reactor 10.

In various embodiments, the in-vessel handling system 28 includes arotating plug 48 and a rotating plug 50 that are both vertically spacedapart from the top of the nuclear fission reactor core 12. The rotatingplug 50 is smaller than the rotating plug 48 and is disposed on top ofthe rotating plug 48. An offset arm machine 52 extends through therotating plug 48 to the top of the nuclear fission reactor core 12. Theoffset arm machine 52 is rotatable through the rotating plug 48. Astraight pull machine 54 extends through the rotating plug 50 to the topof the nuclear fission reactor core 12.

Lower ends of the offset arm machine 52 and the straight pull machine 54include suitable gripping devices, such as grapples or the like, thatenable gripping of selected fissile nuclear fuel assemblies 18 andfertile nuclear fuel assemblies 20 a and 20 b (and in some applications,as will be discussed below, neutron absorber assemblies disposed in theperipheral core region 24) by the offset arm machine 52 and the straightpull machine 54 during movement operations.

Rotation of the rotating plugs 48 and 50 and the offset arm machine 52allows the offset arm machine 52 and the straight pull machine 54 to belocalized to any desired position for pulling a selected assembly out ofthe nuclear fission reactor core 12 and for re-inserting the selectedassembly into the nuclear fission reactor core 12 at any desired emptylocation.

In some embodiments the in-vessel handling system 28 may be furtherconfigured to move ones of the neutron absorber assemblies amongselected locations in the peripheral core region 24. In such cases, thelocations in the peripheral core region 24 may be selected frompredetermined radial locations in the peripheral core region 24 basedupon a predetermined burnup level of nuclear fuel assemblies 18, 20 a,and/or 20 b (depending upon stage of core life and burnup levels) thatare located in the peripheral core region 24. In some other embodiments,the in-vessel handling system 28 may be further configured to rotateones of the neutron absorber assemblies.

In some other embodiments, the in-vessel handling system 28 may befurther configured to shuffle ones of the fissile nuclear fuelassemblies 18 and ones of the fertile nuclear fuel assemblies 20 aand/or 20 b (depending upon stage of core life and burnup levels)between the central core region 16 and a portion of the reactor vessel14 that is located as desired exterior the nuclear fission reactor core12.

In-Core Components

Given by way of nonlimiting overview, in embodiments of the nuclearfission reactor core 12 a sufficient number of fissile nuclear fuelassemblies achieve initial criticality and sufficient breeding toapproach a steady state reactor core breed-and-burn(breeding-and-fissioning) condition. The fissile assemblies areprimarily located in the central core region 16, which generates most ofthe core power. Fertile nuclear fuel assemblies are placed in thecentral core region 16 and the peripheral core region 24 and theirnumber is selected such that reactor operation is possible for up to 40years or more without the need to bring new fuel into the reactor. Theinitial core loading is configured to achieve criticality with a smallamount of excess reactivity and ascension to full power output shortlyafter initial reactor startup. Excess reactivity increases because ofbreeding until a predetermined burnup is achieved in a selected numberof fuel assemblies. The reactivity increase is compensated by movablereactivity control assemblies, which are gradually inserted into thecore to maintain core criticality.

Still given by way of non-limiting overview, a wave of breeding andfissioning (a “breed-burn wave” is originated in the central core region16 but does not move through fixed core material. Instead, a “standing”wave of breeding and fissioning (“burning”) is established byperiodically moving core material in and out of the breed-burn region.This movement of fuel assemblies is referred to as “fuel shuffling” andwill be described in more detail later.

Details regarding components within the nuclear fission reactor core 12will now be discussed by way of non-limiting examples. When relevant,differences over core life in composition and/or burnup levels of fuelassemblies and/or locations of fuel assemblies within the nuclearfission reactor core 12 will be noted.

Regardless of stage of core life, the central core region 16 includesthe movable reactivity control assemblies 22. The movable reactivitycontrol assemblies 22 suitably may be provided as control rods and maybe moved axially in and/or out of the central core region 16 byassociated control rod drive mechanisms. It will be appreciated thataxial position of the movable reactivity control assemblies 22 may beadjusted by the control rod drive mechanisms to insert neutron absorbingmaterial into the central core region 16 and/or to remove neutronabsorbing material from the central core region 16 as desired (such asto compensate for increases in reactivity, to compensate for decreasesin reactivity, to shut down the reactor for planned shutdowns, and/or tostart up the reactor after the reactor has been shut down). It will alsobe appreciated that in some embodiments the movable reactivity controlassemblies 22 may perform safety functions, shut as rapidly insertingneutron absorbing material to rapidly shut down the reactor (that is,scramming the reactor). In some embodiments, neutron absorbing materialdisposed in the movable reactivity control assemblies 22 may includehafnium hydride.

Also regardless of stage of core life, the peripheral core region 24includes the neutron absorber assemblies 26. Unlike the movablereactivity control assemblies 22 (which may be moved during reactoroperation as desired, such as to compensate for increases inreactivity), the neutron absorber assemblies 26 remain in-place and donot move during reactor operation. The neutron absorber assemblies 26help maintain a low core power level in the peripheral core region 24.This low power level helps to simplify coolant flow requirements in theperipheral core region 24. This low power level also helps to mitigatefurther increases in burnup in fuel assemblies that previously have beenused for power production in the central core region 16 and subsequentlyhave been moved from the central core region 16 to the peripheral coreregion 24. In some embodiments, neutron absorbing material disposed inthe neutron absorber assemblies 26 may include hafnium hydride.

However, as discussed above, in some embodiments, if desired the neutronabsorber assemblies 26 may be moved by the in-vessel handling system 28among selected locations in the peripheral core region 24. As mentionedabove, the locations in the peripheral core region 24 may be selectedfrom predetermined radial locations in the peripheral core region 24based upon a predetermined burnup level of nuclear fuel assemblies 18,20 a, and/or 20 b (depending upon stage of core life and burnup levels)that are located in the peripheral core region 24. As also discussedabove, in some other embodiments the neutron absorber assemblies 26 maybe rotated by the in-vessel handling system 28.

Now that the movable reactivity control assemblies 22 and the neutronabsorber assemblies 26 have been discussed, the nuclear fuel assemblies18, 20 a, and 20 b will be discussed. As mentioned above, thisdiscussion includes references to various stages of core life.

Regardless of stage of core life, fertile material in the fertilenuclear fuel assemblies 20 (that is, the fertile nuclear fuel assemblies20 a and the fertile nuclear fuel assemblies 20 b) includes U²³⁸. Invarious embodiments, the U²³⁸ may include natural uranium and/ordepleted uranium. Thus, in various embodiments at least one of thefertile nuclear fuel assemblies 20 a may include U²³⁸ that includesnatural uranium. In some other embodiments, at least one of the fertilenuclear fuel assemblies 20 a may include U²³⁸ that includes depleteduranium. In some embodiments, at least one of the fertile nuclear fuelassemblies 20 b may include U²³⁸ that includes natural uranium. In someembodiments, at least one of the fertile nuclear fuel assemblies 20 bmay include U²³⁸ that includes depleted uranium. That is, at any pointin core life any one or more of the nuclear fuel assemblies 20 a mayinclude U²³⁸ that includes natural uranium, any one or more of thenuclear fuel assemblies 20 a may include U²³⁸ that includes depleteduranium, any one or more of the nuclear fuel assemblies 20 b may includeU²³⁸ that includes natural uranium, and/or any one or more of thenuclear fuel assemblies 20 b may include U²³⁸ that includes depleteduranium.

Thus, regardless of stage of core life, the U²³⁸ in the fertile nuclearfuel assemblies 20 a and/or 20 b need not be limited to any one ofnatural uranium or depleted uranium. Therefore, at any stage in corelife, one or more of the nuclear fuel assemblies 20 a may includenatural uranium, one or more of the nuclear fuel assemblies 20 a mayinclude depleted uranium, one or more of the nuclear fuel assemblies 20b may include natural uranium, and/or one or more of the nuclear fuelassemblies 20 b may include depleted uranium.

At beginning of life (BOL), in various embodiments the central coreregion 16 includes the fissile nuclear fuel assemblies 18, the fertilenuclear fuel assemblies 20 a, and the movable reactivity controlassemblies 22, and the peripheral core region includes the fertilenuclear fuel assemblies 20 b and the neutron absorber assemblies 26. Thefertile nuclear fuel assemblies 20 a and 20 b, the movable reactivitycontrol assemblies 22, and the neutron absorber assemblies 26have beendiscussed above for all stages of core life, including BOL.

At BOL, the central core region 16 includes the fissile nuclear fuelassemblies 18 and the fertile nuclear fuel assemblies 20, and duringcore life (and possibly at end-of-life) the central core region 16includes the fissile nuclear fuel assemblies 18 and the fertile nuclearfuel assemblies 20 a and/or 20 b. The nuclear fuel assemblies 18 and 20may be arranged as desired within the central core region 16. In someembodiments, the nuclear fuel assemblies 18 and 20 may be arrangedsymmetrically within the central core region 16.

At BOL, the fissile nuclear fuel assemblies 18 include enriched fissilenuclear assemblies 18 a. In various embodiments, enriched fissilematerial in the enriched fissile nuclear assemblies 18 a includes U²³⁵.Uranium in the enriched fissile nuclear fuel assemblies 18 a istypically enriched less than twenty percent (20%) in the U²³⁵ isotope.It will be appreciated that in some embodiments (such as the first of afleet of the nuclear fission reactors 10), at BOL all of the fissilematerial in the fissile nuclear fuel assemblies 18 a includes U²³⁵.

However, in other embodiments (such as in later nth-of-a-kind members ofa fleet of the nuclear fission reactors 10), as will be discussed belowat BOL at least some of the fissile material in the fissile nuclear fuelassemblies 18 a may include Pu²³⁹ (that has been bred in previousmembers of the fleet of nuclear fission reactors 10).

It will be further appreciated that only a small mass of fissile nuclearfuel material (relative to the total mass of nuclear fuel material,including fertile nuclear fuel material, included in the nuclear fissionreactor core 10 and, as will be appreciated, as opposed to aconventional fast breeder reactor) is entailed in initiating abreeding-and-fissioning (breed-burn) wave in the nuclear fission reactorcore 10. Illustrative initiation and propagation of abreeding-and-fissioning (breed-burn) wave is described by way of exampleand not of limitation in U.S. patent application Ser. No. 11/605,943 ,entitled AUTOMATED NUCLEAR POWER REACTOR FOR LONG-TERM OPERATION, namingRODERICK A. HYDE, MURIEL Y. ISHIKAWA, NATHAN P. MYHRVOLD, AND LOWELL L.WOOD, JR. as inventors, filed 28 Nov. 2006 , the contents of which arehereby incorporated by reference. It will further be noted that it iswithin the capacity of a person of skill in the art of nuclear fissionreactor design and operation to determine, without undueexperimentation, the amount of fissile nuclear fuel material that isentailed in initiating a breeding-and-fissioning (breed-burn) wave in anuclear fission reactor core 10 of any size as desired.

It will also be appreciated that a breed-burn wave does not move throughfixed core material. Instead, a “standing” wave of breeding and burning(fissioning) is established by periodically moving core material in andout of the breed-burn region. This movement of fuel assemblies isreferred to as “fuel shuffling” and will be described in more detaillater.

It will be appreciated that after BOL the nuclear fission reactor 10 hasbeen started up and the enriched fissile nuclear fuel assemblies 18 ahave begun fissioning. Some of the neutrons may be absorbed by nuclei offertile material, such as U²³⁸, in the fertile nuclear fuel assemblies20 a in the central core region 16. As a result of such absorption, insome instances the U²³⁸ will be converted via capture to U²³⁹, then viaβ decay to Np²³⁹, then via further β decay to Pu²³⁹. Thus, in such casesthe fertile material (that is, U²³⁸) in the fertile nuclear fuelassemblies 20 a will have been bred to fissile material (that is, Pu²³⁹)and, as a result, such fertile nuclear fuel assemblies 20 a will havebeen converted into bred nuclear fuel assemblies 18 b.

Therefore, it will be appreciated that after BOL the fissile nuclearfuel assemblies 18 in the central core region 16 include the enrichedfissile nuclear fuel assemblies 18 a and the bred fissile nuclear fuelassemblies 18 b. As discussed above, fissile material in the enrichedfissile nuclear fuel assemblies 18 a may include U²³⁵ and fissilematerial in the bred fissile nuclear fuel assemblies 18 b may includePU²³⁹.

Some of the other neutrons may be absorbed by other nuclei of fertilematerial, such as U²³⁸, in the fertile nuclear fuel assemblies 20 a inthe central core region 16. As a result of such absorption, in someother instances it will be appreciated that the U²³⁸ in some of thefertile nuclear fuel assemblies 20 a may undergo fast fission.

It will be further appreciated that, after BOL, some neutrons may leakfrom the central core region 16 to the peripheral core region 24. Insuch cases, some of the leaked neutrons may be absorbed by fertilematerial (such as U²³⁸) in the fertile nuclear fuel assemblies 20 b inthe peripheral core region 24. As a result of such absorption and asdiscussed above, in some instances the U²³⁸ will be converted viacapture to U²³⁹, then via β decay to Np²³⁹, then via further β decay toPu²³⁹. Thus, in such cases the fertile material (that is, U²³⁸) in thefertile nuclear fuel assemblies 20 b will have been bred to fissilematerial (that is, Pu²³⁹) and, as a result, such fertile nuclear fuelassemblies 20 b will have been converted into bred nuclear fuelassemblies 18 b. Thus, in such cases, after BOL the peripheral coreregion 24 may include ones of the bred fissile nuclear fuel assemblies18 b.

Some of the other leaked neutrons may be absorbed by other nuclei offertile material, such as U²³⁸, in the fertile nuclear fuel assemblies20 b in the peripheral core region 24. As a result of such absorption,in some other instances it will be appreciated that the U²³⁸ in some ofthe fertile nuclear fuel assemblies 20 b may undergo fast fission. Asdiscussed above, the neutron absorber assemblies 26 help maintain a lowpower level in the peripheral core region even though fast fission ofU²³⁸ in the fertile nuclear fuel assemblies 20 b in the peripheral coreregion 24 may occur.

The enriched fissile nuclear fuel assemblies 18 a will undergo burnupafter BOL. After some time after BOL, the enriched fissile nuclear fuelassemblies 18 a will accumulate sufficient burnup such that it will bedesired to shuffle (or move) such enriched fissile nuclear fuelassemblies 18 a from the central core region 16 to the peripheral coreregion 24 (with the in-vessel handling system 28). It will beappreciated that a person of skill in the art of nuclear fission reactordesign and operation will be able to determine, without undueexperimentation, a burnup level at which one of the enriched fissilenuclear fuel assemblies 18 a is to be shuffled from the central coreregion 16 to the peripheral core region 24. Thus, in such cases, theperipheral core region 24 may further include selected ones of theenriched fissile nuclear fuel assemblies 18 a having at least apredetermined burnup level.

Likewise, the bred fissile nuclear fuel assemblies 18 b will alsoundergo burnup after BOL. After some time after BOL, the bred fissilenuclear fuel assemblies 18 b will accumulate sufficient burnup such thatit will be desired to shuffle (or move) such bred fissile nuclear fuelassemblies 18 b from the central core region 16 to the peripheral coreregion 24 (with the in-vessel handling system 28). It will beappreciated that a person of skill in the art of nuclear fission reactordesign and operation will be able to determine, without undueexperimentation, a burnup level at which one of the enriched fissilenuclear fuel assemblies 18 b is to be shuffled from the central coreregion 16 to the peripheral core region 24. Thus, in such cases, theperipheral core region 24 may further include selected ones of the bredfissile nuclear fuel assemblies 18 b having at least a predeterminedburnup level.

It will further be appreciated that, as discussed above, some of thefertile nuclear fuel assemblies 20 b in the peripheral core region 24will be converted to the bred fissile nuclear fuel assemblies 18 b. Asalso discussed above, the fertile nuclear fuel assemblies 20 b may havebeen subject to neutron flux levels in the peripheral core region 24below neutron flux levels in the central core region 16 to which thefertile nuclear fuel assemblies 20 a have been subjected. As a result,the peripheral core region 24 may include ones of the bred fissilenuclear fuel assemblies 18 b (that is, converted from the fertilenuclear fuel assemblies 20 b in the peripheral core region 24) havingless than a predetermined burnup level.

During various stages of core life, ones of the neutron absorberassemblies 26 may be moved by the in-vessel handling system 28 among anyof several locations in the peripheral core region 24. The locations inthe peripheral core region 24 may include predetermined radial locationsin the peripheral core region 24 that are selectable based upon apredetermined burnup level of nuclear fuel assemblies 18 and 20 that arelocated in the peripheral core region 24.

Toward end-of-life (EOL), the enriched fissile nuclear fuel assemblies18 a may have undergone sufficient burnup such that the enriched fissilenuclear fuel assemblies 18 a have been shuffled (moved) from the centralcore region 16 to the peripheral core region 24. Thus, toward EOL thefissile nuclear fuel assemblies 18 that are located in the central coreregion 16 are the bred fissile nuclear fuel assemblies 18 b. Therefore,toward EOL, the fissile nuclear fuel assemblies 18 (in the central coreregion 16) include the bred fissile nuclear fuel assemblies 18 b, andthe peripheral core region 24 includes enriched fissile nuclear fuelassemblies 18 a having at least a predetermined burnup level.

It will be appreciated that, toward EOL, the peripheral core region mayalso include bred fissile fuel assemblies 18 b. Some of the bred fissilenuclear fuel assemblies 18 b in the peripheral core region 24 mayinclude selected ones of the bred fissile nuclear fuel assemblies 18 bthat have been shuffled from the central core region 16 to theperipheral core region 24 and that have at least a predetermined burnuplevel. It will further be appreciated that some others of the bredfissile nuclear fuel assemblies 18 b in the peripheral core region 24may include (i) ones of the bred fissile nuclear fuel assemblies 18 bthat have been shuffled from the central core region 16 to theperipheral core region 24 that have less than a predetermined burnuplevel and/or (ii) ones of the bred fissile nuclear fuel assemblies 18 bthat have been converted from ones of the fertile nuclear fuelassemblies 20 b (that have resided in the peripheral core region 24)that have less than a predetermined burnup level.

Embodiments of the nuclear fission reactor 10 lend themselves to fuelrecycling. Some embodiments of the nuclear fission reactor 10 maydischarge their fuel at an average burnup of approximately 15% ofinitial heavy metal atoms, with axial peaking making the peak burnup inthe range of 28-32%. Meanwhile, fissile material bred in variousembodiments of the nuclear fission reactor 10 of nominal ‘smear’composition may remain critical to over 40% average burnup (even withoutany fission product removal) via melt refining. Including the effect ofperiodic melt refining can allow burn-ups exceeding 50% to be achieved.Therefore, fuel discharged from a first generation nuclear fissionreactor 10 still has most of its potential life remaining from aneutronic standpoint (even before any “life extension” associated withthermal removal of fission products during recladding is considered) andwould be available for re-use without any need for chemicalreprocessing.

To that end and as mentioned above, in some embodiments (such as inlater nth-of-a-kind members of a fleet of the nuclear fission reactors10), at BOL at least some of the fissile material in the fissile nuclearfuel assemblies 18 a may include Pu²³⁹ (that has been bred in previousmembers of the fleet of nuclear fission reactors 10). In some suchcases, one or more of the fissile nuclear fuel assemblies 18 may includefissile material that has been discharged from a nuclear fissionreactor. Moreover, in some of these cases the fissile nuclear fuelassemblies 18 that include fissile material that has been dischargedfrom a nuclear fission reactor may include re-clad fissile fuelassemblies.

In such embodiments, the fissile nuclear fuel assemblies 18 may berecycled via fuel recladding—a process in which the old clad is removedand the used fuel is refabricated into new fuel. The fissile fuelmaterial is recycled through thermal and physical (but not chemical)processes. The used fuel assemblies are disassembled into individualfuel rods which then have their cladding mechanically cut away. The usedfuel then undergoes a high temperature (1300-1400° C.) melt refiningprocess in an inert atmosphere which separates many of the fissionproducts from the fuel in two main ways: (i) the volatile and gaseousfission products (e.g., Br, Kr, Rb, Cd, I, Xe, Cs) simply escape; while(ii) the more than 95% of the chemically-reactive fission products(e.g., Sr, Y, Te, Ba, and rare earths) become oxidized in a reactionwith the zirconia crucible and are readily separated. The melt-refinedfuel can then be cast or extruded into new fuel slugs, placed into newcladding with a sodium bond, and integrated into new fuel assemblies.

Referring additionally to FIG. 3, an illustrative nuclear fuel assembly(regardless of whether it is a fissile nuclear fuel assembly 18 or afertile nuclear fuel assembly 20) includes fuel pins (or fuel rods orfuel elements) 56. In various embodiments, the fuel pins 56 includemetal fuel (again, regardless of whether the fuel is fissile fuel orfertile fuel). It will be appreciated that metal fuel offers high heavymetal loadings and excellent neutron economy, which is desirable for thebreed-and-burn process in the nuclear fission reactor core 12.

In various embodiments the metal fuel may be alloyed with about 3% toabout 8% zirconium to dimensionally stabilize the alloy duringirradiation and to inhibit low-temperature eutectic and corrosion damageof the cladding. A sodium thermal bond fills the gap that exists betweenthe alloy fuel and the inner wall of the clad tube to allow for fuelswelling and to provide efficient heat transfer which keeps the fueltemperatures low. Individual fuel pins 56 may have a thin wire 58 fromabout 0.8 mm diameter to about 1.6 mm diameter helically wrapped aroundthe circumference of the clad tubing to provide coolant space andmechanical separation of individual fuel pins 56 within the housing ofthe fuel assembly 18 and 20 (that also serves as the coolant duct). Invarious embodiments the cladding, wire wrap, and housing may befabricated from ferritic-martensitic steel because of its irradiationperformance as indicated by a body of empirical data.

Large power differences between the fissile nuclear fuel assemblies 18in the central core region 16 and the fertile nuclear fuel assemblies 20a and/or 20 b in the peripheral core region 24 entail significantdifferences in assembly flow distribution to match flow to power andthus outlet temperature. In various embodiments this flow distributionis accomplished through orifices, such as a combination of fixed andvariable orifices, which make it possible to optimize primary coolantflow proportionally to predicted assembly power.

Referring now to FIG. 4A, in various embodiments orifices 60, such asfixed orifices, are installed in fuel assembly flow receptacles 62 belowthe nuclear fission reactor core 12. The fuel assembly flow receptacles62 mate with seats 64 in a core support grid plate 66 and containsockets 68 where the nuclear fuel assemblies 18 and 20 are inserted.

The fuel assembly flow receptacles 62 have orifices 60 that may be usedto match flow to power generated in the nuclear fuel assemblies. Forexample, the fuel assembly flow receptacles 62 under the peripheral coreregion 24 have very high-pressure-drop orifices 60 to minimize the flowinto very low-power fertile nuclear fuel assemblies 20. On the otherhand, the fuel assembly flow receptacles 62 below the nuclear fuelassemblies 18 and 20 in the central core region 16 may be divided intoseveral groups having orifices 60 ranging from very low resistance tohigher resistance to match the radial power profile in the central coreregion 16.

In addition to the fixed orifices 60, in some embodiments each nuclearfission fuel assembly 18 and 20 may have an ability to adjust assemblyflow by rotation during fuel shuffling operations to enable minor flowadjustments at the assembly level, if desired.

Thus, in some embodiments, the fuel assembly flow receptacles 62 maydefine a group of reactor coolant flow orifices 60 in the central coreregion 16 and another group of reactor coolant flow orifices 60 in theperipheral core region 24. The group of reactor coolant flow orifices 60in the central core region 16 may includes reactor coolant flow orificegroups. In such cases, flow rate through a selected one of the reactorcoolant flow orifice groups may be based upon a power profile at aradial location of the selected one of the reactor coolant flow orificegroups. Moreover, flow rate through the reactor coolant flow orifices 60in the peripheral core region 24 may include a predetermined flow ratebased upon power level in the peripheral core region 24.

In various embodiments, the orifices 60 include fixed orifices. In otherembodiments, variable orifices may be provided (via rotation of thenuclear fuel assemblies 18 and 20). In some other embodiments, theorifices 60 may include fixed orifices and variable orificing may alsobe provided (via rotation of the nuclear fuel assemblies 18 and 20).

In some other embodiments and referring additionally to FIG. 4B, a coresupport grid plate 66 a may be “stepped”. That is, the stepped coresupport grid plate 66 a may be used to offset the nuclear fuelassemblies 18 and 20 axially. As such, the stepped core support gridplate 66 a allows changing position of the nuclear fuel assemblies 18and 20 in the axial direction as a function of their position in theradial direction.

Utilization of fuel in the nuclear fission reactor core 12 may befurther increased by offsetting the assemblies axially (in addition toshuffling the nuclear fuel assemblies 18 and 20 radially). It will beappreciated that relative neutron flux distribution is higher in thecentral axial zone of the nuclear fission reactor core 12 than in theaxial extents of the nuclear fission reactor core 12, as shown by curve67. Such axially offsetting can allow for fuel bred near the axialextents of the fertile nuclear fuel assemblies 20 to be moved closer to(or, if needed, further from) the central axial zone of the nuclearfission reactor core 12. Such offsetting can thus allow for a higherdegree of control of bum-up in the axial dimension, which can furtherhelp yield higher fuel utilization.

In some embodiments the stepped core support grid plate 66 a may includea single axially-sectioned assembly. In some embodiments the level ofoffset could be fixed and could include a pre-determined fuel managementstrategy. In some other embodiments the level of offset may be alteredthrough the use of spacers, such as risers or shims or the like, thatmay be installed at the bottom of the nuclear fuel assemblies 18 and 20or directly onto the stepped core support grid plate 66 a.

Aspects of operation of embodiments of the nuclear fission reactor core12 will be explained.

It will be appreciated that various design features of embodiments ofthe nuclear fission reactor core 10 can help increase the maximum burnupand fluence the fuel can sustain before the accumulation of fissionproducts makes the fuel subcritical.

For example, the fissile nuclear fuel assemblies 18 in the central coreregion 16 are surrounded by subcritical feed fuel (that is, the fertilenuclear fuel assemblies 20 in the central core region 16 and in theperipheral core region 24), which absorbs leakage neutrons and uses themto breed new fuel. It will be appreciated by those of skill in the artof nuclear reactor design and operation that past a thickness of feedfuel surrounding the central core region 16 of approximately 70 cm (or,depending upon size of the fertile nuclear fuel assemblies 20, about 5assembly rows) the fraction of neutrons leaking from the nuclear fissionreactor core 12 is reduced toward zero.

Such neutron conserving features accomplish two things. First, theyminimize the burnup and fluence entailed in achievingbreeding-and-fissioning wave propagation, which in turn eases materialdegradation issues and enables embodiments of the nuclear fissionreactor 10 to be made with existing materials. Second, they increase themaximum burnup and fluence the fuel can sustain before the accumulationof fission products makes the fuel subcritical. This second point isillustrated in FIG. 6A.

Referring additionally to FIG. 6A, a graph 70 graphs reactivity versusburnup for illustrative embodiments of the nuclear fission reactor core12 along a curve 72. The graph 70 compares the reactivity evolution offeed fuel in illustrative embodiments of the nuclear fission reactorcore 12 (illustrated along the curve 72) with the reactivity evolutionof enriched fuel from a typical sodium fast reactor which is illustratedalong a curve 74. The enriched fuel from a typical sodium fast reactoris modeled as having SuperPhenix fuel, coolant and structure volumefractions with 75% smear density, and an initial enrichment of 16%. Asis known, typical sodium fast reactor fuel must start at a highenrichment to achieve criticality, and the excess reactivity of freshfuel is lost to control elements, absorption in the breeding blanket,and leakage from the core. As shown by the curve 74, the typical sodiumfast reactor fuel quickly loses reactivity as U²³⁵ is depleted, and itbecomes subcritical at approximately 310 MWd/kgHM burnup. At the pointwhere the typical sodium fast reactor fuel becomes subcritical, abouthalf of the total fissions are due to U²³⁵, and the utilization fractionof U²³⁸ is less than 20%.

Meanwhile, as shown by the curve 72, feed fuel in embodiments of thenuclear fission reactor core 12 begins as subcritical fertile fuel inthe fertile nuclear fuel assemblies 20 and gains reactivity as Pu²³⁹ isbred in. Once the fuel becomes critical, excess reactivity is offset bybreeding additional subcritical feed fuel (it will be noted that duringthe first 50 MWd/kgHM of burn-up, the driver fuel makes the reactorcritical). A total fuel burnup of up to 400 MWd/kgHM or higher can beachieved before the fuel becomes subcritical, and since the fuel beginsas nearly all U²³⁸, the U²³⁸ utilization fraction can be greater than40%.

Referring additionally to FIG. 6B, a graph 76 of reactivity versusburnup shows effects of periodic thermal removal of fission productsalong a curve 78. The graph 76 also includes the graph 72 for feed fuelwithout thermal removal of fission products. Embodiments of the nuclearfission reactor core 12 are presently designed to discharge their fuelat an average burnup of approximately 15% of initial heavy metal atoms,with axial peaking making the peak burnup in the range of 28-32%.Meanwhile, as shown by the curve 72, feed fuel bred in an illustrativenuclear fission reactor core 12 of nominal ‘smear’ composition remainscritical to over 40% average burnup, even without any fission productremoval via melt refining. Including the effect of periodic meltrefining, as shown by the curve 78, allows burn-ups exceeding 50% to beachieved. Therefore, fuel discharged from a first generation nuclearfission reactor 10 still has most of its potential life remaining from aneutronic standpoint (even before any “life extension” associated withthermal removal of fission products during recladding is considered) andwould be available for reuse without any need for chemical reprocessing.

Referring now to FIG. 7, a graph 80 illustrates plutonium isotopeevolution versus utilization of U²³⁸. At low utilization, the plutoniumproduced is substantially all Pu²³⁹, since operation begins with U²³⁸and no plutonium. At higher utilizations, the plutonium quality becomesincreasingly degraded as higher isotopes of plutonium are created. Atthe point which the feed fuel's k-infinity falls below unity (as shownby the curve 72 in FIGS. 6A and 6B), the fissile plutonium fraction isunder 70%, similar to reactor-grade plutonium from LWR spent fuel.Additionally, the plutonium in spent fuel from embodiments of thenuclear fission reactor 10 is contaminated to a much higher degree withfission products, thereby making it more difficult to handle andreprocess and less attractive for diversion to weapons purposes.

Illustrative Methods

Following are a series of flowcharts depicting implementations. For easeof understanding, the flowcharts are organized such that the initialflowcharts present implementations via an example implementation andthereafter the following flowcharts present alternate implementationsand/or expansions of the initial flowchart(s) as either sub-componentoperations or additional component operations building on one or moreearlier-presented flowcharts. Those having skill in the art willappreciate that the style of presentation utilized herein (e.g.,beginning with a presentation of a flowchart(s) presenting an exampleimplementation and thereafter providing additions to and/or furtherdetails in subsequent flowcharts) generally allows for a rapid and easyunderstanding of the various process implementations. In addition, thoseskilled in the art will further appreciate that the style ofpresentation used herein also lends itself well to modular and/orobject-oriented program design paradigms.

Given by way of overview and referring now to FIG. 8A, a method 100 isprovided for operating a nuclear fission reactor. The method 100 startsat a block 102. At a block 104 fissile nuclear fuel material isfissioned in a plurality of fissile nuclear fuel assemblies in a centralcore region of a nuclear fission reactor core of a nuclear fissionreactor. At a block 106 fissile material is bred in ones of a pluralityof fertile nuclear fuel assemblies in the central core region of thenuclear fission reactor core. At a block 108 selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies areshuffled in a manner that establishes a standing wave of breedingfissile nuclear fuel material and fissioning fissile nuclear fuelmaterial. The method 100 stops at a block 110. Details will be set forthbelow by way of non-limiting examples.

Referring to FIG. 8B, in some embodiments fissioning fissile nuclearfuel material in a plurality of fissile nuclear fuel assemblies in acentral core region of a nuclear fission reactor core of a nuclearfission reactor at the block 104 may include generating in the centralcore region at least a predetermined amount of power in the nuclearfission reactor core at a block 112.

Referring to FIG. 8C, in some embodiments neutrons may be absorbed in aperipheral core region at a block 114.

Referring to FIG. 8D, in some embodiments absorbing neutrons in aperipheral core region at the block 114 may include absorbing neutronsin others of the plurality of fertile nuclear fuel assemblies in theperipheral core region at a block 116.

Referring to FIG. 8E, in some embodiments absorbing neutrons in othersof the plurality of fertile nuclear fuel assemblies in the peripheralcore region at the block 116 may include breeding fissile material inothers of the plurality of fertile nuclear fuel assemblies in theperipheral core region at a block 118.

Referring to FIG. 8F, in some embodiments absorbing neutrons in aperipheral core region at the block 114 may include absorbing neutronsin a plurality of neutron absorber assemblies in the peripheral coreregion at a block 120.

Referring to FIG. 8G, in some embodiments absorbing neutrons in aplurality of neutron absorber assemblies in the peripheral core regionat the block 120 may include absorbing neutrons in a plurality ofneutron absorber assemblies in the peripheral core region such thatpower produced in the peripheral core region is maintained below apredetermined power level at a block 122.

Referring to FIG. 8H, in some embodiments absorbing neutrons in aperipheral core region at the block 114 may include absorbing neutronsin others of the plurality of fertile nuclear fuel assemblies in theperipheral core region and absorbing neutrons in a plurality of neutronabsorber assemblies in the peripheral core region at a block 124.

Referring to FIG. 8I, in some embodiments at a block 126 the nuclearfission reactor may be shut down before shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies.

Referring to FIG. 8J, in some embodiments shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies in amanner that establishes a standing wave of breeding fissile nuclear fuelmaterial and fissioning fissile nuclear fuel material at the block 108may include shuffling selected ones of the plurality of fissile nuclearfuel assemblies and selected ones and selected others of the pluralityof fertile nuclear fuel assemblies between the central core region andthe peripheral core region in a manner that establishes a standing waveof breeding fissile nuclear fuel material and fissioning fissile nuclearfuel material at a block 128.

Referring to FIG. 8K, in some embodiments shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies atthe block 108 may include replacing selected ones of the plurality offissile nuclear fuel assemblies of the central core region with selectedones of the plurality of fertile nuclear fuel assemblies of the centralcore region and with selected others of the plurality of fertile nuclearfuel assemblies of the peripheral core region at a block 130.

Referring to FIG. 8L, in some embodiments shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies atthe block 108 may include shuffling selected ones of the plurality offissile nuclear fuel assemblies having a predetermined burnup level andselected ones and selected others of the plurality of fertile nuclearfuel assemblies at a block 132.

Referring to FIG. 8M, in some embodiments reactivity in the central coreregion may be controlled at a block 134.

Referring to FIG. 8N, in some embodiments controlling reactivity in thecentral core region at the block 134 may include controlling reactivityin the central core region with a plurality of movable reactivitycontrol assemblies at a block 136.

Referring to FIG. 8O, in some embodiments controlling reactivity in thecentral core region at the block 134 may include shuffling selected onesof the plurality of fissile nuclear fuel assemblies and selected onesand selected others of the plurality of fertile nuclear fuel assembliesat a block 138.

Referring to FIG. 8P, in some embodiments controlling reactivity in thecentral core region at the block 134 may include controlling reactivityin the central core region with a plurality of movable reactivitycontrol assemblies and shuffling selected ones of the plurality offissile nuclear fuel assemblies and selected ones and selected others ofthe plurality of fertile nuclear fuel assemblies at a block 140.

Referring to FIG. 8Q, in some embodiments reactor coolant may be flowedthrough a first plurality of reactor coolant flow orifices in thecentral core region at a block 142 and reactor coolant may be flowedthrough a second plurality of reactor coolant flow orifices in theperipheral core region at a block 144.

Referring to FIG. 8R, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 142 may include flowing reactor coolantthrough a plurality of reactor coolant flow orifice groups in thecentral core region at a block 146. In some embodiments, flow ratethrough a selected one of the plurality of reactor coolant flow orificegroups may be based upon a power profile at a radial location of theselected one of the plurality of reactor coolant flow orifice groups. Insome embodiments, flow rate through the second plurality of reactorcoolant flow orifices may include a predetermined flow rate based uponpower level in the peripheral core region.

Referring to FIG. 8S, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 142 and flowing reactor coolant througha second plurality of reactor coolant flow orifices in the peripheralcore region at the block 144 may include maintaining substantiallysteady flow of reactor coolant through ones of the first and secondpluralities of reactor coolant flow orifices at a block 148.

Referring to FIG. 8T, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 142 and flowing reactor coolant througha second plurality of reactor coolant flow orifices in the peripheralcore region at the block 144 may include varying flow of reactor coolantthrough others of the first and second pluralities of reactor coolantflow orifices at a block 150.

Referring to FIG. 8U, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 142 and flowing reactor coolant througha second plurality of reactor coolant flow orifices in the peripheralcore region at the block 144 may include maintaining substantiallysteady flow of reactor coolant through ones of the first and secondpluralities of reactor coolant flow orifices and varying flow of reactorcoolant through others of the first and second pluralities of reactorcoolant flow orifices at a block 152.

Referring to FIG. 8V, in some embodiments flow of reactor coolant may bevaried through at least one of the shuffled nuclear fuel assemblies at ablock 154.

Referring to FIG. 8W, in some embodiments varying flow of reactorcoolant through at least one of the shuffled nuclear fuel assemblies atthe block 154 may include rotating at least one of the shuffled nuclearfuel assemblies at a block 156.

Referring to FIG. 8X, in some embodiments ones of the plurality ofneutron absorber assemblies may be moved among a plurality of locationsin the peripheral core region at a block 158. In some embodiments, theplurality of locations in the peripheral core region may include aplurality of predetermined radial locations in the peripheral coreregion that are selectable based upon a predetermined burnup level ofones of the fissile nuclear fuel assemblies that have been shuffled intothe peripheral core region.

Referring to FIG. 8Y, in some embodiments at a block 160 ones of theplurality of fissile nuclear fuel assemblies and ones and others of theplurality of fertile nuclear fuel assemblies may be selected forshuffling in a manner that establishes a standing wave of breedingfissile nuclear fuel material and fissioning fissile nuclear fuelmaterial. In some embodiments selecting ones of the plurality of fissilenuclear fuel assemblies and ones and others of the plurality of fertilenuclear fuel assemblies for shuffling in a manner that establishes astanding wave of breeding fissile nuclear fuel material and fissioningfissile nuclear fuel material may be based upon at least one operationaldatum chosen from neutron flux data, fuel assembly outlet temperature,and fuel assembly flow rate.

Given by way of overview and referring now to FIG. 9A, a method 200 isprovided for operating a nuclear fission reactor. The method 200 startsat a block 202. At a block 204 fissile nuclear fuel material isfissioned in a plurality of fissile nuclear fuel assemblies in a centralcore region of a nuclear fission reactor core of a nuclear fissionreactor. At a block 206 fissile material is bred in ones of a pluralityof fertile nuclear fuel assemblies in the central core region of thenuclear fission reactor core. At a block 208 reactivity in the centralcore region is controlled. At a block 210 neutrons are absorbed in aperipheral core region. At a block 212 selected ones of the plurality offissile nuclear fuel assemblies and selected ones and selected others ofthe plurality of fertile nuclear fuel assemblies are shuffled in amanner that establishes a standing wave of breeding fissile nuclear fuelmaterial and fissioning fissile nuclear fuel material. The method 200stops at a block 214. Details will be set forth below by way ofnon-limiting examples.

Referring to FIG. 9B, in some embodiments fissioning fissile nuclearfuel material in a plurality of fissile nuclear fuel assemblies in acentral core region of a nuclear fission reactor core of a nuclearfission reactor at the block 204 may include generating in the centralcore region at least a predetermined amount of power in the nuclearfission reactor core at a block 216.

Referring to FIG. 9C, in some embodiments absorbing neutrons in aperipheral core region at the block 210 may include absorbing neutronsin others of the plurality of fertile nuclear fuel assemblies in theperipheral core region at a block 218.

Referring to FIG. 9D, in some embodiments absorbing neutrons in othersof the plurality of fertile nuclear fuel assemblies in the peripheralcore region at the block 218 may include breeding fissile material inothers of the plurality of fertile nuclear fuel assemblies in theperipheral core region at a block 220.

Referring to FIG. 9E, in some embodiments absorbing neutrons in aperipheral core region at the block 210 may include absorbing neutronsin a plurality of neutron absorber assemblies in the peripheral coreregion at a block 222.

Referring to FIG. 9F, in some embodiments absorbing neutrons in aplurality of neutron absorber assemblies in the peripheral core regionat the block 222 may include absorbing neutrons in a plurality ofneutron absorber assemblies in the peripheral core region such thatpower produced in the peripheral core region is maintained below apredetermined power level at a block 224.

Referring to FIG. 9G, in some embodiments absorbing neutrons in aperipheral core region at the block 210 may include absorbing neutronsin others of the plurality of fertile nuclear fuel assemblies in theperipheral core region and absorbing neutrons in a plurality of neutronabsorber assemblies in the peripheral core region at a block 226.

Referring to FIG. 9H, in some embodiments at a block 228 the nuclearfission reactor may be shut down before shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assembliesbetween the central core region and the peripheral core region.

Referring to FIG. 9I, in some embodiments shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies in amanner that establishes a standing wave of breeding fissile nuclear fuelmaterial and fissioning fissile nuclear fuel material at the block 212may include shuffling selected ones of the plurality of fissile nuclearfuel assemblies and selected ones and selected others of the pluralityof fertile nuclear fuel assemblies between the central core region andthe peripheral core region in a manner that establishes a standing waveof breeding fissile nuclear fuel material and fissioning fissile nuclearfuel material at a block 230.

Referring to FIG. 9J, in some embodiments shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies atthe block 212 may include replacing selected ones of the plurality offissile nuclear fuel assemblies of the central core region with selectedones of the plurality of fertile nuclear fuel assemblies of the centralcore region and with selected others of the plurality of fertile nuclearfuel assemblies of the peripheral core region at a block 232.

Referring to FIG. 9K, in some embodiments shuffling selected ones of theplurality of fissile nuclear fuel assemblies and selected ones andselected others of the plurality of fertile nuclear fuel assemblies atthe block 212 may include shuffling selected ones of the plurality offissile nuclear fuel assemblies having a predetermined burnup level andselected ones and selected others of the plurality of fertile nuclearfuel assemblies at a block 234.

Referring to FIG. 9L, in some embodiments controlling reactivity in thecentral core region at the block 208 may include controlling reactivityin the central core region with a plurality of movable reactivitycontrol assemblies at a block 236.

Referring to FIG. 9M, in some embodiments controlling reactivity in thecentral core region at the block 208 may include shuffling selected onesof the plurality of fissile nuclear fuel assemblies and selected onesand selected others of the plurality of fertile nuclear fuel assembliesat a block 238.

Referring to FIG. 9N, in some embodiments controlling reactivity in thecentral core region at the block 208 may include controlling reactivityin the central core region with a plurality of movable reactivitycontrol assemblies and shuffling selected ones of the plurality offissile nuclear fuel assemblies and selected ones and selected others ofthe plurality of fertile nuclear fuel assemblies at a block 240.

Referring to FIG. 9O, in some embodiments reactor coolant may be flowedthrough a first plurality of reactor coolant flow orifices in thecentral core region at a block 242 and reactor coolant may be flowedthrough a second plurality of reactor coolant flow orifices in theperipheral core region at a block 244.

Referring to FIG. 9P, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 242 may include flowing reactor coolantthrough a plurality of reactor coolant flow orifice groups in thecentral core region at a block 246. In some embodiments flow ratethrough a selected one of the plurality of reactor coolant flow orificegroups may be based upon a power profile at a radial location of theselected one of the plurality of reactor coolant flow orifice groups. Insome embodiments flow rate through the second plurality of reactorcoolant flow orifices may include a predetermined flow rate based uponpower level in the peripheral core region.

Referring to FIG. 9Q, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 242 and flowing reactor coolant througha second plurality of reactor coolant flow orifices in the peripheralcore region at the block 244 may include maintaining substantiallysteady flow of reactor coolant through ones of the first and secondpluralities of reactor coolant flow orifices at a block 248.

Referring to FIG. 9R, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 242 and flowing reactor coolant througha second plurality of reactor coolant flow orifices in the peripheralcore region at the block 244 may include varying flow of reactor coolantthrough others of the first and second pluralities of reactor coolantflow orifices at a block 250.

Referring to FIG. 9S, in some embodiments flowing reactor coolantthrough a first plurality of reactor coolant flow orifices in thecentral core region at the block 242 and flowing reactor coolant througha second plurality of reactor coolant flow orifices in the peripheralcore region at the block 244 may include maintaining substantiallysteady flow of reactor coolant through ones of the first and secondpluralities of reactor coolant flow orifices and varying flow of reactorcoolant through others of the first and second pluralities of reactorcoolant flow orifices at a block 252.

Referring to FIG. 9T, in some embodiments flow of reactor coolantthrough at least one of the shuffled nuclear fuel assemblies may bevaried at a block 254.

Referring to FIG. 9U, in some embodiments varying flow of reactorcoolant through at least one of the shuffled nuclear fuel assemblies atthe block 254 may include rotating at least one of the shuffled nuclearfuel assemblies at a block 256.

Referring to FIG. 9V, in some embodiments ones of the plurality ofneutron absorber assemblies may be moved among a plurality of locationsin the peripheral core region at a block 258. In some embodiments theplurality of locations in the peripheral core region may include aplurality of predetermined radial locations in the peripheral coreregion that are selectable based upon a predetermined burnup level ofones of the fissile nuclear fuel assemblies that have been shuffled intothe peripheral core region.

Referring to FIG. 9W, in some embodiments at a block 260 ones of theplurality of fissile nuclear fuel assemblies and ones and others of theplurality of fertile nuclear fuel assemblies may be selected forshuffling in a manner that establishes a standing wave of breedingfissile nuclear fuel material and fissioning fissile nuclear fuelmaterial. In some embodiments selecting ones of the plurality of fissilenuclear fuel assemblies and ones and others of the plurality of fertilenuclear fuel assemblies for shuffling in a manner that establishes astanding wave of breeding fissile nuclear fuel material and fissioningfissile nuclear fuel material may be based upon at least one operationaldatum chosen from neutron flux data, fuel assembly outlet temperature,and fuel assembly flow rate.

Given by way of overview and referring now to FIG. 10A, a method 300 isprovided for managing excess reactivity in a nuclear fission reactor.The method 300 starts at a block 302. At a block 304, criticality with apositive quantity of reactivity is achieved in a central core region ofa reactor core of a nuclear fission reactor. At a block 306 the quantityof reactivity is increased until a predetermined burnup level isachieved in selected ones of fuel assemblies in the reactor core. At ablock 308 the increase in reactivity is compensated for. The method 300stops at a block 310. Details will be set forth below by way ofnon-limiting examples.

Referring to FIG. 10B, in some embodiments increasing the quantity ofreactivity until a predetermined burnup level is achieved in selectedones of fuel assemblies in the reactor core at the block 306 may includemonotonically increasing the quantity of reactivity until apredetermined burnup level is achieved in selected ones of fuelassemblies in the reactor core at a block 312.

Referring to FIG. 10C, in some embodiments increasing the quantity ofreactivity until a predetermined burnup level is achieved in selectedones of fuel assemblies in the reactor core at the block 306 may includeincreasing amount of fissile material in ones of the fuel assemblies ofthe reactor core until a predetermined burnup level is achieved inselected ones of fuel assemblies in the reactor core at a block 314.

Referring to FIG. 10D, in some embodiments increasing amount of fissilematerial in ones of the fuel assemblies of the reactor core until apredetermined burnup level is achieved in selected ones of fuelassemblies in the reactor core at the block 314 may include breedingfissile fuel material from fertile fuel material at a block 316.

Referring to FIG. 10E, in some embodiments compensating for the increasein reactivity at the block 308 may include inserting neutron absorbingmaterial into the central core region at a block 318.

Referring to FIG. 10F, in some embodiments inserting neutron absorbingmaterial into the central core region at the block 318 may includeinserting control rods into the central core region at a block 320.

Referring to FIG. 10G, in some embodiments inserting neutron absorbingmaterial into the central core region at the block 318 may includereplacing selected fissile fuel assemblies in the central core regionwith fertile fuel assemblies from a peripheral core region of thereactor core at a block 322.

Referring to FIG. 10H, in some embodiments inserting neutron absorbingmaterial into the central core region at the block 318 may includeinserting control rods into the central core region and replacingselected fissile fuel assemblies in the central core region with fertilefuel assemblies from a peripheral core region of the reactor core at ablock 324.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in any Application Data Sheet, are incorporated herein byreference, to the extent not inconsistent herewith.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations are not expressly set forth herein for sakeof clarity.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures may beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected”, or“operably coupled,” to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable,” to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents, and/or wirelessly interactable, and/or wirelesslyinteracting components, and/or logically interacting, and/or logicallyinteractable components.

In some instances, one or more components may be referred to herein as“configured to,” “configured by,” “configurable to,” “operable/operativeto,” “adapted/adaptable,” “able to,” “conformable/conformed to,” etc.Those skilled in the art will recognize that such terms (e.g.“configured to”) can generally encompass active-state components and/orinactive-state components and/or standby-state components, unlesscontext requires otherwise.

While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims are toencompass within their scope all such changes and modifications as arewithin the true spirit and scope of the subject matter described herein.It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to claims containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations). Furthermore, in those instances where a conventionanalogous to “at least one of A, B, and C, etc.” is used, in generalsuch a construction is intended in the sense one having skill in the artwould understand the convention (e.g., “a system having at least one ofA, B, and C” would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). In those instances where aconvention analogous to “at least one of A, B, or C, etc.” is used, ingeneral such a construction is intended in the sense one having skill inthe art would understand the convention (e.g., “a system having at leastone of A, B, or C” would include but not be limited to systems that haveA alone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that typically a disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms unless context dictates otherwise. For example, the phrase “Aor B” will be typically understood to include the possibilities of “A”or “B” or “A and B.”

With respect to the appended claims, those skilled in the art willappreciate that recited operations therein may generally be performed inany order. Also, although various operational flows are presented in asequence(s), it should be understood that the various operations may beperformed in other orders than those which are illustrated, or may beperformed concurrently. Examples of such alternate orderings may includeoverlapping, interleaved, interrupted, reordered, incremental,preparatory, supplemental, simultaneous, reverse, or other variantorderings, unless context dictates otherwise. Furthermore, terms like“responsive to,” “related to,” or other past-tense adjectives aregenerally not intended to exclude such variants, unless context dictatesotherwise.

Those skilled in the art will appreciate that the foregoing specificexemplary processes and/or devices and/or technologies arerepresentative of more general processes and/or devices and/ortechnologies taught elsewhere herein, such as in the claims filedherewith and/or elsewhere in the present application.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A nuclear fission reactor comprising: a reactorvessel; a nuclear fission reactor core disposed in the reactor vessel,the nuclear fission reactor core including: a central core regionincluding: a plurality of fissile nuclear fuel assemblies; ones of aplurality of fertile nuclear fuel assemblies; and a plurality of movablereactivity control assemblies; and a peripheral core region locatedradially outward from the central core region and radially inward of aradial shield of the core, the peripheral core region including: othersof the plurality of fertile nuclear fuel assemblies; and a plurality ofneutron absorber assemblies disposed among the others of the pluralityof fertile nuclear fuel assemblies in the peripheral core region,locations of the plurality of neutron absorber assemblies selected frompredetermined radial locations in the peripheral core region based upona predetermined stage of core life of the others of the plurality offertile nuclear fuel assemblies and any other nuclear fuel assemblies ofthe peripheral core region; an in-vessel handling system configured toshuffle ones of the plurality of fissile nuclear fuel assemblies andones of the plurality of fertile nuclear fuel assemblies; and a reactorcoolant system.
 2. The nuclear fission reactor of claim 1, wherein thein-vessel handling system is further configured to shuffle ones of theplurality of fissile nuclear fuel assemblies and ones of the pluralityof fertile nuclear fuel assemblies between the central core region andthe peripheral core region.
 3. The nuclear fission reactor of claim 1,wherein the plurality of fissile nuclear fuel assemblies includes: aplurality of enriched fissile nuclear fuel assemblies; and a pluralityof bred fissile nuclear fuel assemblies.
 4. The nuclear fission reactorof claim 3, wherein the peripheral core region further includes ones ofthe plurality of bred fissile nuclear fuel assemblies.
 5. The nuclearfission reactor of claim 3, wherein the peripheral core region furtherincludes selected ones of the plurality of enriched fissile nuclear fuelassemblies having at least a predetermined burnup level.
 6. The nuclearfission reactor of claim 3, wherein the peripheral core region furtherincludes selected ones of the plurality of bred fissile nuclear fuelassemblies having at least a predetermined burnup level.
 7. The nuclearfission reactor of claim 3, wherein the peripheral core region furtherincludes ones of the plurality of bred fissile nuclear fuel assemblieshaving less than a predetermined burnup level.
 8. The nuclear fissionreactor of claim 1, wherein: the plurality of fissile nuclear fuelassemblies includes a plurality of bred fissile nuclear fuel assemblies;and the core peripheral region includes a plurality of enriched fissilenuclear fuel assemblies having at least a predetermined burnup level. 9.The nuclear fission reactor of claim 8, wherein the peripheral coreregion further includes ones of the plurality of bred fissile nuclearfuel assemblies.
 10. The nuclear fission reactor of claim 9, wherein theperipheral core region further includes selected ones of the pluralityof bred fissile nuclear fuel assemblies having at least a predeterminedburnup level.
 11. The nuclear fission reactor of claim 9, wherein theperipheral core region further includes ones of the plurality of bredfissile nuclear fuel assemblies having less than a predetermined burnuplevel.
 12. The nuclear fission reactor of claim 1, wherein at least oneof the plurality of fissile nuclear fuel assemblies includes fissilematerial discharged from a nuclear fission reactor.
 13. The nuclearfission reactor of claim 12, wherein the plurality of fissile nuclearfuel assemblies that includes fissile material discharged from a nuclearfission reactor includes at least one re-clad fissile fuel assembly. 14.The nuclear fission reactor of claim 1, further comprising a pluralityof fuel assembly flow receptacles defining: a first plurality of reactorcoolant flow orifices in the central core region; and a second pluralityof reactor coolant flow orifices in the peripheral core region.
 15. Thenuclear fission reactor of claim 14, wherein the first plurality ofreactor coolant flow orifices includes a plurality of reactor coolantflow orifice groups.
 16. The nuclear fission reactor of claim 15,wherein flow rate through a selected one of the plurality of reactorcoolant flow orifice groups is based upon a power profile at a radiallocation of the selected one of the plurality of reactor coolant floworifice groups.
 17. The nuclear fission reactor of claim 14, whereinflow rate through the second plurality of reactor coolant flow orificesincludes a predetermined flow rate based upon power level in theperipheral core region.
 18. The nuclear fission reactor of claim 14,wherein the first and second pluralities of reactor coolant floworifices include fixed orifices.
 19. The nuclear fission reactor ofclaim 14, wherein the first and second pluralities of reactor coolantflow orifices include variable orifices.
 20. The nuclear fission reactorof claim 14, wherein the first and second pluralities of reactor coolantflow orifices include fixed orifices and variable orifices.
 21. Thenuclear fission reactor of claim 1, wherein the in-vessel handlingsystem is further configured to move ones of the plurality of neutronabsorber assemblies among a plurality of locations in the peripheralcore region.
 22. The nuclear fission reactor of claim 21, wherein theplurality of locations in the peripheral core region include a pluralityof predetermined radial locations in the peripheral core region that areselectable based upon a predetermined burnup level of nuclear fuelassemblies located in the peripheral core region.
 23. The nuclearfission reactor of claim 1, wherein the in-vessel handling system isfurther configured to rotate ones of the plurality of neutron absorberassemblies.
 24. The nuclear fission reactor of claim 1, wherein thein-vessel handling system is further configured to shuffle ones of theplurality of fissile nuclear fuel assemblies and ones of the pluralityof fertile nuclear fuel assemblies between the central core region and aportion of the reactor vessel exterior the nuclear fission reactor core.25. The nuclear fission reactor of claim 1, wherein the reactor coolantsystem includes a pool of liquid sodium disposed in the reactor vessel,the nuclear fission reactor core being submerged in the pool of liquidsodium.
 26. The nuclear fission reactor of claim 1, wherein the neutronabsorber assemblies are surrounded by the fertile fuel assemblies of theperipheral core region and inside the radial shield.
 27. The nuclearfission reactor of claim 1, wherein at least a portion of the neutronabsorber assemblies within the peripheral core region are not movable.28. The nuclear fission reactor of claim 1, wherein at a beginning oflife BoL of the nuclear fission reactor core, the peripheral core regionincludes no fissile nuclear fuel assemblies.
 29. A nuclear fissionreactor comprising: a reactor vessel; a nuclear fission reactor coredisposed in the reactor vessel, the nuclear fission reactor coreincluding: a central core region including: a plurality of fissilenuclear fuel assemblies; ones of a plurality of fertile nuclear fuelassemblies; and a plurality of movable reactivity control assemblies;and a peripheral core region located radially outward from the centralcore region and radially inward of a radial shield of the core, theperipheral core region including: others of the plurality of fertilenuclear fuel assemblies; and a plurality of neutron absorber assembliesdisposed among the others of the plurality of fertile nuclear fuelassemblies in the peripheral core region, locations of the plurality ofneutron absorber assemblies selected from predetermined radial locationsin the peripheral core region based upon a predetermined stage of corelife of the others of the plurality of fertile nuclear fuel assembliesand any other nuclear fuel assemblies of the peripheral core region; anin-vessel handling system configured to shuffle ones of the plurality offissile nuclear fuel assemblies and ones of the plurality of fertilenuclear fuel assemblies; a reactor coolant system; a plurality of fuelassembly flow receptacles defining: a first plurality of reactor coolantflow orifices in the central core region; and a second plurality ofreactor coolant flow orifices in the peripheral core region; and a decayheat removal system.
 30. The nuclear fission reactor of claim 29,wherein the in-vessel handling system is further configured to shuffleones of the plurality of fissile nuclear fuel assemblies and ones of theplurality of fertile nuclear fuel assemblies between the central coreregion and the peripheral core region.
 31. The nuclear fission reactorof claim 29, wherein the plurality of fissile nuclear fuel assembliesincludes: a plurality of enriched fissile nuclear fuel assemblies; and aplurality of bred fissile nuclear fuel assemblies.
 32. The nuclearfission reactor of claim 31, wherein the peripheral core region furtherincludes ones of the plurality of bred fissile nuclear fuel assemblies.33. The nuclear fission reactor of claim 31, wherein the peripheral coreregion further includes selected ones of the plurality of enrichedfissile nuclear fuel assemblies having at least a predetermined burnuplevel.
 34. The nuclear fission reactor of claim 31, wherein theperipheral core region further includes selected ones of the pluralityof bred fissile nuclear fuel assemblies having at least a predeterminedburnup level.
 35. The nuclear fission reactor of claim 31, wherein theperipheral core region further includes ones of the plurality of bredfissile nuclear fuel assemblies having less than a predetermined burnuplevel.
 36. The nuclear fission reactor of claim 29, wherein: theplurality of fissile nuclear fuel assemblies includes a plurality ofbred fissile nuclear fuel assemblies; and the core peripheral regionincludes a plurality of enriched fissile nuclear fuel assemblies havingat least a predetermined burnup level.
 37. The nuclear fission reactorof claim 36, wherein the peripheral core region further includes ones ofthe plurality of bred fissile nuclear fuel assemblies.
 38. The nuclearfission reactor of claim 37, wherein the peripheral core region furtherincludes selected ones of the plurality of bred fissile nuclear fuelassemblies having at least a predetermined burnup level.
 39. The nuclearfission reactor of claim 37, wherein the peripheral core region furtherincludes ones of the plurality of bred fissile nuclear fuel assemblieshaving less than a predetermined burnup level.
 40. The nuclear fissionreactor of claim 29, wherein at least one of the plurality of fissilenuclear fuel assemblies includes fissile material discharged from anuclear fission reactor.
 41. The nuclear fission reactor of claim 40,wherein the plurality of fissile nuclear fuel assemblies that includesfissile material discharged from a nuclear fission reactor includes atleast one re-clad fissile fuel assembly.
 42. The nuclear fission reactorof claim 29, wherein the first plurality of reactor coolant floworifices includes a plurality of reactor coolant flow orifice groups.43. The nuclear fission reactor of claim 42, wherein flow rate through aselected one of the plurality of reactor coolant flow orifice groups isbased upon a power profile at a radial location of the selected one ofthe plurality of reactor coolant flow orifice groups.
 44. The nuclearfission reactor of claim 29, wherein flow rate through the secondplurality of reactor coolant flow orifices includes a predetermined flowrate based upon power level in the peripheral core region.
 45. Thenuclear fission reactor of claim 29, wherein the first and secondpluralities of reactor coolant flow orifices include fixed orifices. 46.The nuclear fission reactor of claim 29, wherein the first and secondpluralities of reactor coolant flow orifices include variable orifices.47. The nuclear fission reactor of claim 29, wherein the first andsecond pluralities of reactor coolant flow orifices include fixedorifices and variable orifices.
 48. The nuclear fission reactor of claim29, wherein the in-vessel handling system is further configured to moveones of the plurality of neutron absorber assemblies among a pluralityof locations in the peripheral core region.
 49. The nuclear fissionreactor of claim 48, wherein the plurality of locations in theperipheral core region include a plurality of predetermined radiallocations in the peripheral core region that are selectable based upon apredetermined burnup level of nuclear fuel assemblies located in theperipheral core region.
 50. The nuclear fission reactor of claim 29,wherein the in-vessel handling system is further configured to rotateones of the plurality of neutron absorber assemblies.
 51. The nuclearfission reactor of claim 29, wherein the in-vessel handling system isfurther configured to shuffle ones of the plurality of fissile nuclearfuel assemblies and ones of the plurality of fissile nuclear fuelassemblies between the central core region and a portion of the reactorvessel exterior the nuclear fission reactor core.
 52. The nuclearfission reactor of claim 29, wherein the reactor coolant system includesa pool of liquid sodium disposed in the reactor vessel, the nuclearfission reactor core being submerged in the pool of liquid sodium.